Lesson objective: Understand that a linear model is often used to describe the relationship in a set of bivariate data.

Students bring prior knowledge of using scatter plots to describe patterns of association from 8.SP.A.1. This prior knowledge is extended to generating a linear model as students find a line of best fit. A conceptual challenge students may encounter is understanding that there may not be exactly one correct answer.

The concept is developed through work with a line through a set of bivariate data on a scatter plot, which helps to explain that a line of best fit is one with about half of the points above the line and half of the points below the line and with all points as close to the line as possible.

This work helps students deepen their understanding of equivalence because we can think about a line of best fit as being a line with an equal number of points above the line as well as below the line.

Students engage in Mathematical Practice 3 (Construct viable arguments and critique the reasoning of others) as they decide which line of best fit is most appropriate for a given scatter plot.

**Key vocabulary:**

- dependent variable
- independent variable
- line of best fit
- linear association
- negative linear association
- positive linear association